The present invention relates to de-icing systems, and particularly to systems which prevent either water vapor or entrained water droplets in compressed air from freezing and clogging up compressed air lines or devices which use compressed air, or both, in low temperature environments. The invention also relates to a novel turbine which is particularly well suited for the de-icing system.
Many mechanical devices use compressed air as a source of power. Other devices use compressed air in other ways for their operation. For example, artificial snow making devices use compressed air to atomize water and distribute the artificial snow over a ski area. Many compressed air systems are designed to be operated in low temperature environments. For example, a sawmill may use compressed air as a power supply source, and the machinery may be located in unheated buildings and used during the winter. Many other outdoor operations, including construction sites, quarries, railroads and marine applications, use compressed air. Air naturally contains water vapor, measured by the term "humidity". When the air is compressed, the water vapor is also compressed. As the compressed air is cooled while under pressure, some of the water vapor condenses into water droplets. If the compressed air is moving, the water droplets often remain entrained or suspended in the moving air stream. As a result, the compressed air thus includes water vapor and entrained water droplets.
There are a number of systems and processes for removing water from compressed air, either using an after-cooler and a separator to remove entrained water droplets, or desiccant dryer to remove water vapor, or both. However, it is usually not cost efficient to remove all of the moisture from the air. This remaining moisture (water vapor or entrained water droplets or both) can then freeze up in the air supply lines, when operating below 32.degree. F., or in the devices that use compressed air when the expansion of the compressed air causes the temperature of the air to fall below freezing.
One solution to preventing water in compressed air lines from freezing is to add a de-iceant to the air. The de-iceant combines with the water and lowers the freezing point of the resulting mixture, much as antifreeze works in a cooling system. Most de-iceants are alcohol-based. However, these cannot be used in some situations, such as in underground mining operations or other confined areas, because the alcohol is combustible and toxic. Other less frequently used de-iceants are propylene glycol-based which are less toxic and non-combustible. One problem with these de-iceants, however, is that they are more viscose, and thus harder to effectively add to compressed air.
One procedure for adding de-iceants is to have a container filled with de-iceant connected to the compressed air line with a venturi system that draws the de-iceant from the container as the compressed air flows past. This system does not atomize the de-iceant. Venturi systems work well on small air lines, up to two inches in diameter or less than 1000 cfm air flow. However, they require repeated refilling of small storage containers. If there are numerous air lines, such venturi systems require the containers filled with de-iceant to be scattered around the compressed air system, with the associated labor-intensive requirement of replenishing the contents of the containers. For larger systems, a larger de-iceant storage tank is desirable, with a controllable valve for introducing de-iceant into the air stream. For instance, a vaporizer may be used to heat the de-iceant to a vapor state, in which it is injected into the compressed air lines. However, a heated vaporizer is not suitable for propylene glycol-based de-iceants.
Another consideration in adding de-iceant is control over the amount of de-iceant added to the compressed air. The optimum amount of de-iceant is dependent on a number of variables, including the moisture content of the compressed air, the flow rate of the compressed air and the ambient temperature. Since these variables can and do change, particularly the flow rate of the compressed air, the optimum amount of de-iceant to add changes. Venturi and other tank systems are generally provided with some control features, and inherently change the feed rate as the flow rate changes. However, for larger compressed air systems using a vaporizer, typical vaporization units do not have a way of automatically changing supply rates as the compressed air flow rate changes. Thus, one adding de-iceant at a constant rate must either add an amount to meet the highest air usage, which would be wasteful when not operating at peak demand, or face potential freeze ups if an insufficient amount is added and the air usage rate goes up. Since the cost of unclogging frozen air lines and the associated down time of operating equipment is so great, operators tend to use more de-iceant than is needed.
Thus, there is a need for a de-icing system which can controllably add a de-iceant to a high volume compressed air stream, preferably a de-iceant that is non-combustible and less toxic than alcohol based de-iceants. It would also be beneficial if the system were capable of monitoring the flow rate of compressed air and automatically changing the rate of addition of de-iceant.